Hot Corrosion Behavior of Single-Layered Gd2Zr2O7, Sm2Zr2O7, and Nd2Zr2O7 Thermal Barrier Coatings Exposed to Na2SO4 + MgSO4 Environment

Zirconates of rare earth elements have emerged as promising candidates for thermal barrier coatings (TBC). This study investigates the hot corrosion resistance of single-layered ceramic coatings composed of Gd2Zr2O7, Sm2Zr2O7, and Nd2Zr2O7. The coatings were prepared using air plasma spraying and applied to an Inconel [IN] 625 substrate. Experimental assessments were conducted to examine the hot corrosion behaviour by subjecting the coatings to pure magnesium sulfate (MgSO4) salt at 1000 °C for 24 h and a 50/50 mole percent Na2SO4 and MgSO4 mixture at 900 °C for cyclic durations of 5, 10, 15, and 20 h. This combination of salts creates a highly corrosive environment. This short test was carried out due to the necessity of the initial stages of the destruction process characterization. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS) techniques were utilized to identify and analyse the reaction products. At 1000 °C, no chemical reaction products were observed between MgSO4 and Gd2Zr2O7, Sm2Zr2O7, and Nd2Zr2O7. However, in the presence of the MgSO4 + Na2SO4 mixture, the zirconate coatings reacted, resulting in the formation of reaction products such as Gd(SO4)3, Gd2O2SO4, Gd2O3, Sm2O2SO4, Sm2(SO4)3, Sm2O3, MgO, Nd2(SO4)3, Na2O, and m-ZrO2. These compounds are formed due to the interaction of rare earth oxides with a low-temperature-melting eutectic Na2SO4+ (3MgSO4 × Na2SO4) melted at 666 °C. Despite the aggressive nature of the corrosive environment, the decomposition of rare earth zirconates was relatively limited, indicating satisfactory resistance to hot corrosion. Among the zirconate systems studied, Gd2Zr2O7 exhibited the lowest resistance to the MgSO4 + Na2SO4-based corrosive environment, while Sm2Zr2O7 and Nd2Zr2O7 demonstrated better corrosion resistance.